Complex machinery, such as plastic injection molding equipment, require precise synchronised interaction between moving parts. Due to space limitations, manufacturing costs and a desire to reduce maintenance, simplicity is highly desirable yet often elusive in many complex machine designs.
A typical design response is to add independently powered pneumatic or hydraulic actuators to provide precise synchronised motions. The synchronised motion of various parts which in the past were accomplished through mechanical means are now increasingly achieved through electronic controllers, sensors and computer programming
A simple mechanical motion transfer device previously developed for synchronising product transfer with opening and closing of a plastic mold, provided an elongate track with a spiral actuating surface. This prior art device is fully described in the applicant's co-pending U.S. patent application Ser. No. 08/577,648 filed Dec. 22, 1995, now U.S. Pat. No. 5,681,595. Rollers following the actuating surface were mounted in a track rider to transfer longitudinal motion of the openable mold components to rotational motion. The rotational motion operated a reciprocating product transfer device between the open mold faces.
This simple motion transfer mechanism provided an inexpensive simple means to synchronise plastic molding production without reliance on relatively expensive pneumatic actuators and electronic controls.
From experience, several disadvantages have arisen in respect of this type of device. The use of hard steel rollers to follow a spiral path on a hardened steel track actuating surface results in extremely high wear and vibration. The vibration and wear effectively render the device impractical in some applications since downtime and parts replacement may eliminate the advantages achieved from the simplicity of the device.
The rollers, riding rapidly on the actuating surface, encounter severe impact loads when they engage the spiral actuating surface and force the track rider to rotate quickly. The rapid impact of the rollers engaging the spiral transition in many cases damages rollers, bearings and produces excessive wear on the track. Such impact damage and extreme wear reduces machine accuracy, reliability and increases downtime. Cylindrical rollers riding on a flat planar actuating surface encounter a geometry problem when the planar actuating surface to spiral actuating surface transition occurs. Easing the transition from planar to spiral surface by axially lengthening the transition or slowing the mold opening and closing speed will reduce impact on the rollers. However, this solution causes a significant problem in an increased mold cycle time and lower overall rate of production.
Close engagement between the actuating surface of the track and rollers of the track rider is essential to accurate and reliable machine synchronisation. Due to wear and impact damage, the clearance between the track and rollers may be rendered unpredictable.
Inaccuracy in machining of the track and assembly of the machine may result in further operation problems. Clearance between the rollers and track is essential for smooth operation, however, inaccuracies may produce jamming or loose engagement. The accuracy of the machine operation is thus compromised.
It is desirable therefore to produce a simple mechanical motion transfer device which can compensate for some inaccuracy in construction, and which can withstand the impact of operation.